By default, WordPress always compressed images upon upload. In version 4.5, WordPress changed Jpeg compression from 90Q to 82Q. This reduced image weight by an average of 25 percent.
Since version 2.5, WordPress used a default-quality setting of 90 to optimize images. A 90Q setting creates larger file sizes than recommended by modern web best practices. For example, WebPagetest warns site owners if images aren’t compressed enough.
WebPagetest specification: Images compressed within 10% of a Photoshop-web-save quality of 50 will pass. Up to 50% larger will warn (we guess that would be around 70Q to 75Q). Anything larger than that will fail. The image score is the percentage of bytes saved by image re-compression.
Photoshop provides a handy “Save for Web” option that keeps file sizes low. But what about a large website with a lot of images? An online store might have thousands of images. Having to create different sizes of each of these is an enormous task.
WordPress uses ImageMagick, an open-source command-line graphics editor, to quickly resizes images. This technique maintains great visual quality and small file sizes. It automates resizing image compression with formats like Jpeg and PNG.
The Odd Number of 82Q.
Researchers compared ImageMagick’s various compression settings against Photoshop’s high quality (60Q) setting for JPEGs. An Imagick compression setting of 82Q was closest. They used an open source test to compare dissimilarity of input and output Jpeg images.
Changing the default image quality setting is a small change. Yet, it makes a big impact on file size without sacrificing perceived image quality. The new 82Q setting only applies to the intermediate size images. Not the original files uploaded by users.
To change original image size dimensions and quality settings, we recommend Imsanity plugin (100,000+ installs).
We repeat: original full-sized, uploaded images remain unaffected. This change only affects images resized by WordPress, such as image thumbnails. For international users, on slower web connections, this is important for performance optimization.
We agree with these changes for improving WordPress speed.